Methods for the production of methanol by catalytic conversion of synthesis gas containing hydrogen and carbon oxides have long since been known to those skilled in the art. For example in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, Chapter “Methanol”, Sub-chapter 5.2 “Synthesis”, a method for the production of methanol is described, which is illustrated in FIG. 1 in a schematic and simplified form. In this Figure, a synthesis gas stream containing hydrogen and carbon oxides is supplied to a compressor 2 via conduit 1 and by said compressor brought to the reaction pressure of typically 5 to 10 MPa. Compressor 2 and compressor 15 can technically be coupled with each other. Via conduit 3, the compressed synthesis gas stream is supplied to a heat exchanger 4 and in the same brought to the reaction temperature, wherein the heat exchange mostly is effected against the hot product gas stream from the synthesis reactor (not shown in FIG. 1). Via conduit 5, the preheated synthesis gas stream enters into the synthesis reactor 6, where at temperatures between 200 and 300° C. the partial conversion of hydrogen with carbon oxides is effected on a methanol synthesis catalyst, wherein a product mixture containing the synthesis gas is obtained. Via conduit 7, the product mixture is discharged from the synthesis reactor. After cooling in the heat exchanger 8, the product mixture flows through conduit 9 into the separator 10, where methanol is separated as crude methanol and supplied to the further product processing via conduit 11. The gas product obtained in the separator is discharged via conduit 12 and separated into a purge stream, which is discharged via conduit 13, and into a cycle stream, which is supplied to the cycle compressor 15 via conduit 14. Via the purge stream, inert components are discharged from the process. Via conduit 16, the cycle stream is recirculated to the synthesis reactor 6, wherein fresh synthesis gas is supplied via conduit 17 and combined with the cycle stream. The ratio of the molar flow rates of cycle stream to fresh gas stream is referred to as cycle ratio.
A more advanced, two-stage method for the production of methanol is described in EP 0 790 226 B1. The methanol is produced in a cyclic process in which a mixture of fresh and partly reacted synthesis gas first is supplied to a water-cooled reactor and then to a gas-cooled reactor, in each of which the synthesis gas is converted to methanol on a copper-based catalyst. The methanol produced in the process is separated from the synthesis gas to be recirculated, which then is countercurrently passed through the gas-cooled reactor as coolant and preheated to a temperature of 220 to 280° C., before it is introduced into the first synthesis reactor. A part of the synthesis gas to be recirculated is removed from the process as purge stream, in order to prevent that inert components are enriched within the synthesis cycle. This measure is also described in the unexamined German Patent Application DE 2934332 A1 and in the European Patent Application EP 1016643 A1.
In the two methods described above it is disadvantageous that when processing synthesis gases with a high content of inert components the cycle ratio must be increased, as due to the lower partial pressures of the reactants the conversion to methanol per passage through the synthesis reactor is lower than in synthesis gas low in inerts. This leads to an increase of the required compressor capacity and—with a given production capacity for methanol—to greater dimensions for apparatuses and conduits.
Inert components on the one hand include inorganic gas constituents such as nitrogen or inert gases, which are obtained for example from the production of synthesis gas proceeding from natural gas with corresponding constituents. Such natural gases are obtained for example from Asian deposits. On the other hand, non-converted methane, which during the gasification of natural gas can be contained in the synthesis gas product, is regarded as an inert gas in the sense of the methanol synthesis. Furthermore, a nitrogen-containing synthesis gas is obtained when the gasification of natural gas is carried out with air or air enriched with oxygen, as it is described for example in the International Patent Application WO 96/14279 A1.
The problems of processing of synthesis gases rich in inerts in the methanol synthesis have long since been known. Various technical solutions have already been proposed, which could, however, not gain acceptance due to their disadvantages.
In the unexamined German Patent Application DE 1296133 B it is proposed, for example, to treat the raw synthesis gas containing inert components such as nitrogen, methane or argon by a xylene wash, whereby distinct reductions of the contents of the inert components should be achieved. Here, it is disadvantageous that before entry into the gas scrubber the temperature of the synthesis gas must be lowered to −10 to −30° C., in order to significantly lower the partial pressures of the inert components. This results in a high loss of energy. In addition, a laden absorbent is obtained, which must be aftertreated and which contains the component xylene foreign to the process of the methanol synthesis.
A similar technical teaching can be taken from the unexamined German Patent Application DE 10156092 A1, in which it is proposed to provide an absorption stage upstream of each catalytic reaction system for producing methanol, which contains a methanol synthesis catalyst as absorbent and which is operated at a temperature which lies below the temperature for the catalytic conversion to methanol. As absorbent an auxiliary substance inherent to the process is employed, but the two above-mentioned disadvantages of a required decrease in temperature and an aftertreatment or disposal of the absorbent still exist.
A method for the production of methanol from a synthesis gas obtained from the autothermal gasification of natural gas, containing 20 to 50% each of hydrogen, carbon monoxide and methane, is described in the patent application EP 1819653 A1. There are not taken any particular measures with respect to the methane remaining in the reactor product of the methanol synthesis reactor, but the hydrogen content is increased by means of conversion, the hydrogen—possibly with the likewise produced carbon dioxide—then is separated and again charged to the methanol synthesis reactor. The partial pressures of the reactants are increased by this measure, but the cycle stream remains high due to the high content of inert components.
In general, it should therefore be noted that no satisfactory technical solution of the problem has been found so far, although the problem, as illustrated, already exists for quite some time. In addition, many of the above discussed methods aim at reducing the purge stream containing inert components by a corresponding treatment as far as possible.